The present invention relates to a coding arrangement including a variable-length encoder having a quality control to determine different coding characteristics, the arrangement comprising an input for receiving information components to be encoded and which are provided in the form of packets of information blocks to be applied to said coder, a buffer store receiving the coded information outputed by the said coder and to produce them at an output terminal thereof with a fixed rate, and a regulating circuit for defining for each block a quality as a function of on the one hand the real number of bits utilized for coding each block and on the other hand a residual quantity which defines the number of bits contained in the buffer store after coding of each packet.
Such an arrangement is advantageously used more specifically in the field of digital television. Thus, each packet can represent a picture and each block a zone of this picture containing a set of picture elements.
This is described in the European Patent Application no. 0.310 175. To profit fully from the advantages of the variable-length encoder, it is advantageous to utilize a large-capacity buffer store which can thus take up considerable inequalities in the length of the encoded words. However, when one wants to record such encoded images on magnetic tape, it is a requirement that each picture is coded with a substantially constant number of bits so as to occupy a fixed space on the magnetic tape. This facilitates the fast search of a recorded sequence of pictures. The known arrangements are therefore limited to the use of a buffer store whose capacity depends on the tolerance permitted to this space. This is of necessity a low capacity.